Multiphoton infrared photoinduced ion-molecule reactions in the gas phase

In the mature area of condensed-phase photochemistry, a limitless number of examples exist in which a species, activated by absorption of a photon, undergoes a reaction not observed in its ground state. In contrast, under the conditions used to study gas-phase ion-molecule chemistry at low pressures (10[sup [minus]5]-10[sup [minus]9] Torr), examples of photoinduced or photoenhanced reactions have been surprisingly sparse. This work more than doubles the number of reported examples in the literature, further demonstrating the potential of this new type of photochemistry. A survey is presented involving organometallic ions in which multiphoton infrared absorption is used to create long-lived vibrationally excited ground-state intermediates which exhibit some dramatic changes in reactivity. In two instances, sustained off-resonance (rf) irradiation, or SORI, which also adds energy in small increments, is shown to mimic the infrared results. The five general reaction types demonstrated are (1) deceleration of exothermic reactions; (2) photoinduced secondary fragmentation, whereby an internally excited product ion is generated which can undergo further fragmentation; (3) photoactivated ligand switching; (4) photoinduced isomerization; and (5) photoenhanced hydrogen/deuterium exchange. 49 refs., 7 figs., 2 tabs.